BUILD: makefile: remove the special case of the SSL option

By creating USE_SSL and enabling it when USE_OPENSSL is set, we can
get rid of the special case that was made with it regarding cflags
collect and when resetting options. The option doesn't need to be
manually set, though in the future it might prove useful if other
non-openssl API are supported.
diff --git a/Makefile b/Makefile
index a5ddc70..b2ff189 100644
--- a/Makefile
+++ b/Makefile
@@ -298,7 +298,7 @@
            USE_THREAD USE_PTHREAD_EMULATION USE_BACKTRACE                     \
            USE_STATIC_PCRE USE_STATIC_PCRE2 USE_TPROXY USE_LINUX_TPROXY       \
            USE_LINUX_SPLICE USE_LIBCRYPT USE_CRYPT_H USE_ENGINE               \
-           USE_GETADDRINFO USE_OPENSSL USE_OPENSSL_WOLFSSL USE_LUA            \
+           USE_GETADDRINFO USE_OPENSSL USE_OPENSSL_WOLFSSL USE_SSL USE_LUA    \
            USE_ACCEPT4 USE_CLOSEFROM USE_ZLIB USE_SLZ USE_CPU_AFFINITY        \
            USE_TFO USE_NS USE_DL USE_RT USE_LIBATOMIC USE_MATH                \
            USE_DEVICEATLAS USE_51DEGREES                                      \
@@ -578,6 +578,7 @@
     SSL_CFLAGS    := $(if $(SSL_INC),-I$(SSL_INC))
     SSL_LDFLAGS   := $(if $(SSL_LIB),-L$(SSL_LIB)) -lssl -lcrypto
   endif
+  USE_SSL         := $(if $(USE_SSL),$(USE_SSL),implicit)
   OPTIONS_OBJS += src/ssl_sock.o src/ssl_ckch.o src/ssl_sample.o src/ssl_crtlist.o src/cfgparse-ssl.o src/ssl_utils.o src/jwt.o src/ssl_ocsp.o
 endif